With the rapid development of the digital communication system, it is highly required for the reliability of data communication; however, in a bad channel environment, especially in a high data rate or high-speed mobile environment, the multi-path interference, the Doppler Shift and the like will severely affect the system performance. Therefore, an effective error control technique, especially the HARQ (Hybrid Automatic Repeat Request) technique becomes a hot research topic in the communication field.
In the HARQ mode, codes transmitted by a transmitting end are not only able to detect errors, but also have a certain error correction capability. After receiving codes, a decoder at a receiving end firstly detects errors: if the errors are within the error correction capability of the codes, they are automatically corrected; if there are too many errors, which go beyond the error correction capability of the codes, but can still be detected, then the receiving end transmits a decision signal to the transmitting end via a feedback channel to ask the transmitting end to retransmit the information. In an OFDM (Orthogonal Frequency Division Multiplexing) system, an ACK/NACK (Acknowledged/Non-Acknowledged) control signal is used to indicate whether the transmission is right or wrong, thereby judging whether the information needs to be re-transmitted or not.
When the ACK/NACK signal is not transmitted together with uplink data, the ACK/NACK signal will be transmitted on a special Physical Uplink Control Channel (PUCCH). In the present LTE (Long Term Evolution) system, the ACK/NACK signal is carried on a PUCCH reference signal structure, which is as shown in FIG. 1.
FIG. 1 illustrates a PUCCH channel reference signal structure in the present LTE system, which is mainly used to carry ACK/NACK information. In this illustration, a sub-frame of 1 ms is divided into two time slots of 0.5 ms, and when the PUCCH uses a conventional cyclic prefix, each time slot of 0.5 ms includes 7 symbols which are numbered as #0˜#6 respectively, and each symbol occupies 12 sub-carriers on the frequency domain; wherein, the symbols numbered as #2˜#4 are used to transmit reference signals, as shown in the “time domain DFT covering” area in FIG. 1, and the four symbols numbered as #0, #1, #5, and #6 are used to transmit information symbols, as shown in the “time domain Walsh covering” area.
For an FDD (Frequency Division Duplex) system, since uplink and downlink sub-frames correspond to each other one by one, the ACK/NACK information transmitted in an uplink sub-frame is 1 bit or 2 bits, which corresponds to one or two streams transmitted downlink respectively. The ACK/NACK information of 1 bit/2 bits forms a modulation symbol after BPSK/QPSK modulation. This modulation symbol is firstly subjected to a frequency spreading with a spreading factor of 12 (a CAZAC sequence with a spreading sequence of 12) in a frequency domain, and then is subjected to a time domain spreading through a Walsh code with a length of 4 in a time domain, and then is mapped to an information symbol corresponding to the PUCCH channel reference signal structure as shown in FIG. 1, and finally forms, together with the reference signal, a signal to be transmitted in a time slot. Therefore, for an FDD system, transmitting the ACK/NACK signal using the PUCCH channel reference signal structure as shown in FIG. 1 can satisfy the performance of the ACK/NACK and the requirements of the coverage.
FIG. 2 illustrates the structure of ACK/NACK channel code resources in the present LTE system. The ACK/NACK signals of the UEs in the same cell are multiplexed through a code division. Firstly, on the frequency domain (in the vertical direction in the figure), each UE is multiplexed through different cyclic shifts of the same CAZAC sequence, and then on the time domain (in the horizontal direction in the figure), the multiplexing capacity is further increased through a Walsh sequence spreading. The PUCCH channel occupies 12 sub-carriers on the frequency domain, thus a CAZAC sequence with a length of 12 is used for the frequency domain spreading. There are 4 information symbols on the time domain, thus a Walsh sequence with a length of 4 is used for the time domain spreading of the information symbol; while there are 3 symbols for a reference signal on the time domain, the DFT (Discrete Fourier Transform) spreading with a length of 3 is performed for the reference signal, as shown in FIG. 2. Generally, a CAZAC sequence with a length of 12 has 12 cyclic shifts available in total, but in view of the delayed spreading of a channel, a UE using the same Walsh sequence of the time domain spreading only uses a part of the 12 available cyclic shifts, for example, only (0, 2, 4, 6, 8, 10) or (1, 3, 5, 7, 9) are used.
However, for a TDD (Time Division Duplex) system, due to the asymmetry of the uplink and downlink, for example, when the sub-frames transmitted on the downlink are more than those transmitted on the uplink, it may be necessary to transmit ACK/NACK signals corresponding to a plurality of downlink transmissions in an uplink sub-frame. When the number of the ACK/NACK information bits to be fed back in an uplink sub-frame is more than or equal to 3 bits, a solution is as follows if the PUCCH channel reference signal structure of FIG. 1 is used for transmission: a plurality of ACK/NACK signals are independently encoded and are transmitted on the same time-frequency resource using different code resources. However, the main problem of this solution is that the transmitted signals are not single-carrier signals any more, and thus will have relatively high PAPR (Peak Average Power Ratio), which is not preferable to a UE (User Equipment).
Therefore, it is necessary to provide a new method for transmitting a plurality of ACK/NACK signals in an uplink sub-frame for a TDD system to solve the above problem.